Known in the art is an internal combustion engine which is provided with a variable compression ratio mechanism which can make a cylinder block move along a cylinder axis with respect to a crankcase so as change the mechanical compression ratio. In general, the lower the engine load, the lower the thermal efficiency, so in such an internal combustion engine which is provided with a variable compression ratio mechanism, the mechanical compression ratio is made higher the lower the engine load so as to raise the expansion ratio and raise the thermal efficiency.
In an internal combustion engine which is provided with a variable compression ratio mechanism as well, it has been proposed to use a turbocharger which utilizes exhaust energy so as to raise the engine output (see PLT 1). For this purpose, a compressor of a turbocharger is arranged in an engine intake system, a turbine of the turbocharger is arranged in an engine exhaust system, and a wastegate passage which bypasses the turbine is provided. The wastegate passage is provided with a wastegate valve. By controlling the opening degree of the wastegate valve, the turbine speed made to change to control the supercharging pressure of the compressor to the desired supercharging pressure.
In such an internal combustion engine which is provided with a variable compression ratio mechanism, a target mechanical compression ratio is set for each current engine operating state and the variable compression ratio mechanism is controlled so that the current target mechanical compression ratio is realized. Further, for the opening degree of the wastegate valve as well, a target opening degree is set so that the desired supercharging pressure for the current engine operating state is realized.
In this regard, an internal combustion engine which is provided with a variable compression ratio mechanism wherein when knocking occurs, the mechanical compression ratio is lowered from the target mechanical compression ratio has also been proposed (see PLT 2).